The invention relates to an electromagnetically actuated mechanical brake for an electric motor, in particular for a geared motor.
Electric machines, in particular electric motors, can be very well controlled with respect to their rotational speed. One known means of improving this basic controllability is to provide mechanical brakes, which can be released or raised by means of an electromagnetic apparatus. While the electric motor is in operation, the current powering the electric motor is also supplied to an excitation coil of the electromagnetically actuated mechanical brake, so that the brake is released and remains released. When no current is thus supplied, the electric motor is immobilized by application of a spring force with a braking action.
Furthermore, a known means of powering electric motors is by an alternating-current or polyphasic mains supply. In this case, if the motor incorporates an excitation coil operated by direct current, converters or rectifiers are employed. Because the operating voltages of a.c. or polyphasic mains vary in different parts of the world, excitation coils are manufactured and sold in a great variety of types. For each type, in turn, there are one or several types of rectifiers that can be combined with the excitation coil.
During the installation of an electromagnetically actuated mechanical brake, mistakes are often made in that a rectifier is wrongly combined with an excitation coil and with other components of the brake. The consequences are malfunction and defects of the excitation coil and/or the rectifier. Furthermore, often when such defects are noted either only the excitation coil or only the rectifier is replaced. The possibility is sometimes overlooked that because of the defect in one component, the other may also have become nonfunctional. Just as when the brake is newly installed, it can happen that during maintenance a defective rectifier or excitation coil is accidentally replaced by one of a different type. For this reason, too, the replacement part and/or the part that has not been replaced may become nonfunctional immediately after installation.
In addition to a rectifier, other circuit components may be provided by means of which an excitation current is sent to the excitation coil. In particular when an excitation coil driven by an a.c. or polyphasic mains supply is used, it is possible to operate without a rectifier. Even in this case, however, circuit components are customarily used to conduct, and in particular to control, an excitation current to the excitation coil. In this case, too, care must be taken to ensure a correct combination of the circuitry that supplies the excitation current with the excitation coil and with other components of the brake.
The object of the invention is to develop an electromagnetically actuated mechanical brake for an electric motor, in particular for a geared motor, in such a way as to make installation as easy as possible and to ensure that it is correct with respect to the combination of components. In addition, the storage of components for the brake is to be made simple and such that it is easy to keep track of the stock.
In the electromagnetically actuated mechanical brake for an electric motor in accordance with the invention, an excitation coil is provided for the electromagnetic actuation of the brake. In addition, a circuit arrangement is provided to supply an excitation current to the excitation coil. The excitation coil and the circuit arrangement are integrated with one another in a structural unit. Other components are also preferably integrated into the structural unit, such as pressure springs to produce a braking force. Because of this integrated design, incorrect combinations of components are nearly impossible. For example, the external shape of the structural unit can be made such that it can be combined with additional components of the brake in only one way, namely with components that are suitable. The additional components in question here are in particular the disks of a disk brake, in provided with friction surfaces and actuated by the excitation coil. The invention considerably simplifies storage, because now instead of at least two components, namely the excitation coil and the circuit arrangement, only a single structural unit needs to be kept in stock. As a result, there is no further need for elaborate lists from which to determine the right combination of components. Furthermore, both new installation and the installation of replacement parts are simplified. For this reason the time the mechanic must spend on installation can be considerably shortened. The components can be assembled in advance, for example automatically on an assembly line.
Preferably the structural unit comprises a one-piece carrier on which both the excitation coil and the circuit arrangement are disposed. The carrier is preferably so constructed that after the structural unit has been completed, the components cannot be detached from the carrier, or are detachable only with great effort or special tools. A one-piece carrier simplifies the construction and hence the manufacture of the structural unit. Because the components are not detachable or are more difficult to detach from the carrier, an incorrect combination of components is practically ruled out.
In a further development, the carrier comprises a housing that preferably consists of metal, in particular gray cast iron or diecast aluminium. These materials guarantee good thermal conductance, so that local overheating can be avoided. This is advantageous particularly when, during prolonged operation of an electric motor, the excitation coil produces heat that must be dissipated. In any case, however, a housing made in one piece can be so designed as to ensure a uniform temperature distribution and good heat transfer to surrounding components or the ambient air.
In another modification the carrier is made of plastic. With a plastic carrier, weight and material costs can be saved.
To avoid overheating of the circuit arrangement, which in particular comprises temperature-sensitive semiconductor components, the structural unit is preferably so constructed that the heat-conduction resistance between the circuit arrangement and the excitation coil is both considerably larger than the heat-conduction resistance between the circuit arrangement and the carrier and also considerably larger than the heat-conduction resistance between the excitation coil and the carrier. In this case both the excitation coil and the circuit arrangement can release heat into the carrier. In a further development the circuit arrangement can additionally transmit heat to the surroundings or other components of the brake by other means; that is, additional heat-conduction resistances, of the smallest possible dimensions, are present in association with the circuit arrangement. In particular, the movement of the electric motor is also utilized to drive forced convection by means of which the circuit arrangement and/or the excitation coil are cooled.
In particular to simplify the manufacture of an electromagnetically actuated mechanical brake in accordance with the invention, the excitation coil and the circuit arrangement are potted in a block of plastic and/or coated with plastic by spraying.
In a further development of the brake in accordance with the invention, the connector by way of which the circuit arrangement is connected to a current source, in particular an a.c. mains supply, is so constructed that a low heat-conduction resistance can be produced between the circuit arrangement and a mains lead. The mains lead in a variant of this embodiment is fixedly joined to the circuit arrangement. In particular, in order to produce the low heat-conduction resistance the connector comprises a thermal contact surface that is larger than a cross-sectional area of the circuit arrangement, the circuit arrangement being disposed along the thermal contact surface at only a small distance therefrom. Thus the mains lead can account for a considerable proportion of the heat conduction away from the circuit arrangement.
In yet another further development, the heat-conduction resistance between the circuit arrangement and the excitation coil is considerably greater than the heat-conduction resistance between the excitation coil and the surroundings of the structural unit. Therefore, under normal operating conditions, overheating of the circuit arrangement can be avoided.
Preferably the circuit arrangement is connected to a cooling plate, preferably made of aluminium, to conduct heat away. In particular, the circuit arrangement extends substantially two-dimensionally along the cooling plate. In this embodiment the cooling plate also ensures that the heat that needs to be dissipated from the power components of the circuit arrangement is distributed or transported away.
It is useful for a thermal barrier to be positioned between the excitation coil and the circuit arrangement. In one embodiment the thermal barrier consists of a material with low heat conductance, in particular a ceramic material. In another embodiment the material connecting the excitation coil to the circuit arrangement is so formed as to amount to a thermal barrier on account of its dimensions. In particular, this connecting material has the shape of a bridge. If this material bridge is suitably constructed, it alone can ensure a high heat-conduction resistance between the excitation coil and the circuit arrangement. In many applications of the brake in accordance with the invention, no means of thermal decoupling other than such a bridge are needed.